A system for studying gene stability and rearrangement in normal and tumorigenic cells has been devised. A plasmid shuttle vector has been constructed. The plasmid contains sequences derived from a bacterial plasmid, from SV40 virus, and a marker gene, galactokinase, which can be scored in the appropriate bacterial host. This construct will replicate in mammalian cells and bacteria. An experimental protocol was designed in which mammalian cells were infected with the plasmid, replication permitted and then the plasmid DNA extracted from the cells. After purification and elimination of residual infectious DNA, the plasmid is introduced into a bacterial host which permits the detection of the presence or absence of a functional galactokinase gene. With this assay it is possible to quantitatively assess the stability of the plasmid in the mammalian cells. In our first experiments we have used a permanent cell line of African green monkey kidney cells as the mammalian host and find that about 1% of the replicated plasmids have lost a functional galactokinase gene. Analysis of the defective plasmids indicate that molecules both larger and smaller than the starting material appear. Experiments designed to assay the plasmid stability in tumorigenic cells are in progress.